1. Field of the Invention
The present invention relates to a heat treatment apparatus and a heat treatment method.
2. Description of Related Art
A bearing ring forming a rolling bearing used in automobiles, industrial machines, and the like has a raceway portion that comes into relative rolling contact with rolling elements. The raceway portion is likely to be subjected to impact from the rolling elements as a result of the rolling contact therewith. Thus, in connection with manufacture of the bearing ring, various techniques have been proposed which improve mechanical properties of the bearing ring.
For example, a technique has been put to practical use in which bulk quenching (through hardening) and tempering are executed on a steel material formed of high carbon chromium bearing steel in order to increase the hardness of the bearing ring to enhance wear resistance and crushing strength of the bearing ring. However, the steel material subjected to the bulk quenching may have a high hardness and thus a high crushing strength, but the steel material as a whole from the surfaces to the inside thereof is formed mostly of the same structure of tempered martensite. Thus, the steel material contains no low hardness area and tends to be inferior in impact resistance.
To enhance the impact resistance, a technique has also been put to practical use in which carburizing and quenching and tempering are executed on a steel material of case hardened steel. However, the steel material subjected to the carburizing and quenching and the tempering has an improved impact resistance, but surfaces of the steel material have a reduced grain boundary strength due to the carburizing. Thus, the steel material tends to be inferior in crushing strength. Furthermore, secondary quenching allows the crushing strength to be enhanced, but an increase in costs resulting from an increased number of steps is unavoidable.
Japanese Patent Application Publication No. 2013-238274 (JP 2013-238274 A) states that high carbon chromium bearing steel such as SUJ2 is thermally refined, with only the surfaces thereof subjected to induction hardening, to allow the hardness of the surfaces of the steel to be set to 700 Hv or more, while allowing the internal hardness of the steel to be set to a small value of 340 to 490 Hv. As a result, this technique improves the rolling life and impact resistance of the bearing ring. However, the technique disclosed in JP 2013-238274 A fails to enclose the entire periphery of an inner layer portion of the steel material (workpiece) with a high hardness layer. As a result, the low hardness area is sandwiched between the high hardness layers, and it is difficult to sufficiently enhance the crushing strength.
As described above, the conventional technique has difficulty in providing a bearing ring that allows both high impact resistance and high crushing strength to be achieved.